Monroe Britt

There are several general ventilation air filters testing standards used worldwide today. The test methods most used in the United States are those published and maintained by the American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc. (ASHRAE). These are commonly referred to as the ASHRAE 52.1 and ASHRAE 52.2 test methods.

ASHRAE 52.1 testing.

ASHRAE 52.1 has been in use for several decades and has served the general ventilation air filter industry well. One of the shortcomings of ASHRAE 52.1 was the lack of a test method to produce filter efficiencies for particles of specific diameters. A “dust spot efficiency” test method is included that allows a quantitative measure of atmospheric dust removal by the air filter undergoing the test. The majority of atmospheric dust is generally found in the 0.3 to 0.5 micron range.

ASHRAE 52.2 testing.

After the introduction of 52.1, major commercial advances have taken place in particle counter technology, and since the mid-1990s, ASHRAE has worked diligently to develop a test method to produce air filter efficiency as a function of particle size. This new standard was published in 1999 and presents a detailed method to measure the ability of an air filter to remove dust by specific particle sizes. This new procedure made it possible to more critically discern the filter’s ability to remove particles ranging from 0.30 microns to 10 microns. With this new information, it became practical to provide more meaningful information on the filter’s efficiency performance other than Initial, Final, and Average Dust Spot Efficiency as produced by the ASHRAE 52.1 test method. With the abundance of efficiency information now available, it became possible to produce a Minimum Efficiency Reporting Value (MERV) for each filter tested. These values provide a greater insight into the applicability of filters to use in a given situation than the dust spot efficiency values from ASHRAE 52.1 (The author refers the reader to the ASHRAE 52.2 standard or other publications for a full discussion of MERV).

Efficiency by particle size.

With the rapidly increasing adoption of the ASHRAE 52.2 test method, the ASHRAE 52.1 committee has begun the process of removing the dust spot efficiency test requirements from the earlier standard. It is anticipated that the revised standard, without dust spot efficiency, will be submitted for public review in the latter part of 2007. It is realized that many public and private companies, as well as governmental agencies, have long-standing technical and purchasing specifications that reference this procedure. The ASHRAE public review process will provide means for the air filter industry’s opinions and comments to be reviewed by ASHRAE. These opinions and comments will be discussed at the ASHRAE Winter Meeting to be held in New York, January 19-23, 2008.

DOP penetration.

Significant changes in ASHRAE 52.2 are underway now. One of the changes seems relatively straightforward. The ASHRAE 52.2 standard was initially written so as not to duplicate any information presented in ASHRAE 52.1 but to reference the needed information from the earlier standard. An addendum is now on ASHRAE’s Website ( in the public review section that will make the 52.2 standard a “stand-alone” standard with no references to ASHRAE 52.1. All information on dust arrestance and dust holding testing and reporting will be included directly in 52.2. Public comments received will be discussed at the ASHRAE Annual Meeting to be held in Long Beach, Calif., June 23-27, 2007.

VOC (hydrocarbon) analysis.

The more significant change pending in ASHRAE 52.2 is the change in the initial filter conditioning step that is now in the standard. The foreword of ASHRAE 52.2 contains a paragraph that discusses the potential for electrostatic fibrous filter media causing an unsustained increase in the media efficiency. The purpose of this initial conditioning step was to acknowledge in the test procedure that the electrostatic phenomena could produce test results that possibly would not be experienced in field service. After several research projects sponsored by ASHRAE, a more comprehensive filter conditioning step has been developed. This conditioning step, while not perfect, does seem to replicate loss of efficiency over time as seen in some in-service filters, particularly those installed with a high degree of outside air intake. This procedure has also been subjected to a “round robin” test program which resulted in minor changes to some test parameters. In general, good agreement was achieved between the majority of the participating testing laboratories.


The proposed filter conditioning step is presented in the public review section of the ASHRAE Website. This new procedure consists of challenging the test filter with sub-micron potassium chloride particles in high concentrations for an extended period of time until the particle size efficiencies for the 12 different particle size bands do not decrease beyond a certain criteria. All filters, regardless of type of media used in their construction, will be subjected to this conditioning step prior to the dust loading and particle size efficiency phases of the test. This revised test procedure has the potential of causing a shifting/substitution of medias that are now used in various MERV level filters. The public review comments for this addendum will also be discussed at the ASHRAE Annual Meeting in Long Beach.

Flat sheet testing.

Another major development on the international level is the reactivation of a working group to establish an ISO standard for testing of general ventilation air filters. This working group under ISO/TC 142 WG03 has held several international meetings and has published a draft of an international standard (ISO/DIS 21220) to solicit comments. The next international meeting will be held in Beijing in November 2007, during which public comments will be discussed and reconciled. Information on this draft standard may be obtained from the Institute of Environmental Sciences and Technology (IEST) Website ( IEST is the administrator of the ANSI-accredited US Technical Advisory Group (TAG) to ISO/TC142. The original basic for this test method was the EN 779 test procedure. During the meetings, the procedure has been modified to include major aspects of the ASHRAE 52.2 test method. As a matter of note, ISO has several working groups addressing test methods for other aspects of air filtration, notably HEPA/ULPA filters, gas phase filters, gas turbine air intake filters, and residential filters. Contact IEST for additional information.


Much activity and progress in general ventilation air filter testing methods are expected in the next several months. The publication and use of these new or revised test standards will require all testing laboratories to modify their test facilities to accommodate these procedures.